DES MOINES — One candidate made an appearance with the world’s largest tractor. Another showed up with the Duggars, the nation’s most famous large family. There were two Pauls in Des Moines and six Romneys in Davenport.

This is Iowa, the day before the circus leaves town.

On the eve of the first balloting of the 2012 presidential primary season, six Republican contenders made their last pitches to voters here, with the three leaders elbowing one another for a finish-line advantage.

If the freshest polls are to be believed, three very different candidates are the front-runners heading into caucus night — although in a campaign as muddled as this one, it’s anyone’s guess who will come out on top. Leading into the final stretch are former Massachusetts governor Mitt Romney, Rep. Ron Paul (Tex.) and former senator Rick Santorum (Pa.).

The other contenders are already looking beyond Tuesday night’s caucuses. At one time or another, Texas Gov. Rick Perry, Rep. Michele Bachmann (Minn.) and former House speaker Newt Gingrich (Ga.) were considered formidable candidates. But in the closing hours before voting, they were scrambling to spin something respectable from what is likely to be a disappointing evening and to put forward a rationale for continuing through contests in New Hampshire, South Carolina and Florida.

“I don’t think I’m going to win,” Gingrich said Monday, lowering expectations while blaming a barrage of negative ads from groups that support Romney.

But he added: “I think we’ve begun to lay out the themes that will work. I think we’ve seen Romney do his most intense negatives, and we now have had time to think through how to respond.”

The final full day of campaigning before the caucuses felt part county fair, part reality television. Gingrich appeared in the town of Independence with Big Bud, the world’s largest tractor. Santorum’s surprise guests in Polk City were the Duggar family , stars of the TLC hit show “19 Kids and Counting” (previously known as “17 Kids and Counting” and “18 Kids and Counting”).

Romney — along with wife Ann, brother Scott and three of his five sons — logged more than 250 miles in four cities, arguing electability and inevitability.

“We’re gonna win this thing,” Romney told more than 300 supporters at an afternoon rally in Marion, where he received his loudest and most enthusiastic reaction of the past week.

Supporters such as Sen. John Thune (R-S.D.) emphasized the more pragmatic calculation that many Iowa Republicans will be making as they ponder whether to support a front-runner with a moderate record that many conservatives mistrust.

“Think about this question,” said Thune, a favorite of the right, at a chilly morning event at the Davenport fairgrounds. “Who is best equipped to actually win the election in November and to defeat Barack Obama?”

The front-runners represent a stark choice: Romney, the establishment favorite; Paul, the libertarian iconoclast with a young and passionate following; and Santorum, the conservative long shot who is experiencing a late-breaking surge resulting from his own tenacity and the collapse of several rivals.

At a rally at the Marriott in downtown Des Moines, Sen. Rand Paul (R-Ky.) — Ron Paul’s son and a tea party icon — argued for conservatives to follow their hearts.

“There is only one candidate who has never been accused of flip-flopping . . . my father, Ron Paul,” he said in one of five stops that the father-son team made on Monday.

In Polk City, Santorum disputed the notion that Romney’s experience as a corporate executive and turnaround artist had prepared him to govern a nation of deeply divided political interests and entrenched constituencies.

An executive “assigns people who work for them. I can tell you, as a senator, I didn’t work for the president. Congress doesn’t work for the president,” Santorum said. “The American people don’t work for the president. It’s the other way around.”

The evidence of Santorum’s recent surge was obvious: The overwhelming crush of media members at the Polk City stop included reporters from Italy and Australia. Dozens of voters — who two weeks ago probably could have had the candidate to themselves — were pressed out of the restaurant and stood in the cold.

“I’m actually from Polk City,” one said to another as he was unable to squeeze his way inside. “Yeah, we don’t count,” the other responded.

Despite the fire-hazard nature of the crowd, Santorum followed a pattern established through 360 previous Iowa events. He took every question voters asked.

“One more question,” he said after speaking for about 30 minutes. “No?” he said, spotting more hands. “Two? Three more questions.”

And he offered long responses. He said that his first executive order as president would be to ban federal funding for abortion and that U.S. citizens accused of terrorism should have access to lawyers and courts. He also promised to push for a balanced-budget amendment to the Constitution.

For limping candidates such as Perry, Gingrich and Bachmann, a dismal showing Tuesday could set off a chain reaction of bad news. Lower fundraising totals. Less advertising. Disappointments in the upcoming primaries in New Hampshire and South Carolina.

The mix of contenders may also shift. Former Utah governor Jon Huntsman Jr., another lagging candidate hoping to catch fire, did not compete in Iowa so that he could focus on New Hampshire.

In Sioux City, Perry — whose weak debate performances demolished the high expectations that had greeted his candidacy — drew on his experience as an avid runner to frame the race ahead.

“This is the first, let’s say, Mile One of the marathon, and I’ve run a marathon before,” he said Monday at a meet-and-greet at the Stoney Creek Inn. “We’ll see who’s still running at Mile 21. I finished my marathon, and I expect to finish this marathon as well.”

A Des Moines Register poll over the weekend indicated that four out of 10 caucus-goers are still open to changing their minds about whom to support. For some, that decision could come down to the final appeals they hear from their neighbors during the quirky, quadrennial exercise that will take place in 1,774 precincts across the state — in schoolhouses, libraries, churches and homes.

“It’s hard. You hear good and bad about them all,” said Marilyn Walker, 75, a retired farmer from Indianola who attended an event for Santorum on Monday evening at a Pizza Ranch in Altoona. “We probably won’t make up our minds until the very last moment.”

She said she and her husband had ruled out Romney, calling him “a waverer.”

They were considering Santor­um, Paul and Bachmann. “We’re going to decide based on integrity and morality.”

After hearing Santorum, Walker said she remained undecided. “It’s food for thought,” she said.